A relay is a type of electronic control device that often used as an automatic control circuit. It has a control system (also known as the input circuit) and a controlled system (also known as the output circuit). A smaller current in the control system can control a larger current of the controlled system using such an “Automatic Switch.”
While simple direct current (DC) relays are common, alternating-current (AC) relays may be somewhat more complex since the AC current switches direction. However, many commercial appliances use AC and thus could benefit from an AC relay to control the AC current that powers the appliance. Electric irons, toasters, and other small electronic appliances can benefit from the use of improved AC relays. The improved AC relays can increase reliability and precision of temperature control for such appliances.
Current products commonly use electromagnetic relays (EMR) and solid state relays (SSR). Electromagnetic relays (EMR) use a mechanical contact switch. The contact resistance and power dissipation of an EMR is very small. However, EMR's have some drawbacks.
The process of switching an EMR may take a few milliseconds to a few tens of milliseconds. When AC current is used, it is difficult to turn the EMR on and off at the zero-crossing of the AC current, when the AC current switches direction. Also, the mechanical contact in the EMR might arc, producing a reduced contact lifetime. EMR's also have a large electromagnetic interference (EMI).
The stochastic operation time causes the EMR to be incapable of turning on or turning off at exactly the load's AC zero crossing. Instead, the electromagnetic relay's switching action brings a surge current. This surge current creates interference in the electrical grid system, producing an inextricable EMI problem for the electromagnetic relay.
Usually an arc discharge phenomena appears at the moment of switching, when the electromagnetic relay controls a high voltage and a large current flows. An electric spark (arc) appears, and the arc discharge creates electrical wear. This electrical wear is much worse than the mechanical wear on the EMR, producing an electrical lifetime that is far less than the mechanical lifetime of the EMR. Usually the electrical lifetime is about fifty to one hundred thousand times, but the mechanical lifetime is over one million times.
Solid state relays use a solid-state semiconductor such as a Silicon-Controlled-Rectifier (SCR) for the switch function. SCR's produce no arcing and no large electromagnetic interference. However, SCR's operate with about a 1-Volt voltage drop. This 1-volt drop through the SCR is a serious problem, especially for high-power control applications.
Simply combining an electromechanical EMR relay with a solid-state relay (SSR) would likely produce the disadvantages of both. The combined device could have low reliability due to arcing of the EMR, and have the voltage-drop problem of the SSR, along with a high cost.
What is desired is an AC relay that combines the benefits of an electromechanical relay and a solid-state relay while reducing or eliminating the disadvantages of each. A relay that switches AC currents near the AC zero-crossing point is desirable. An AC relay that solves the problems of electromagnetic interference (EMI) and high power dissipation when switching large AC currents is also desirable.